The invention relates to color display systems which employ one or more electro-optic display devices. Such a display device serves as a light modulator, either in the reflective or transmissive mode, to control the grey level of projected light at each pixel point. More particularly, the invention relates to digital to analog (DAC) driver circuitry employed in such a color display system to convert incoming digital display signals to analog signals, and to address the individual pixels of the display device with such analog signals.
Color display systems are known in which light bars of different colors are sequentially scrolled across a single electro-optic light modulator panel to produce a color display. See, for example, commonly assigned U.S. Pat. No. 5,532,763, incorporated herein by reference. These display systems are particularly suitable for displaying color information in the form of continuously updated image information signals arranged in successive frames, such as color video information, in which each frame is composed of component color sub-frames, e.g., red, green and blue sub-frames.
These systems employ an electro-optic light modulator panel comprised of a row-and-column matrix array of pixels, for modulating light in accordance with the image information signals during successive frame periods. The analog signal information is applied to the pixel columns of the array, a row at a time, during each frame period.
A system of this type is also disclosed in the publication of J.A. Shimizu, xe2x80x9cSingle Panel Reflective LCD Projectorxe2x80x9d, Projection Displays V, Proceedings SPIE, Vol. 3634, pp. 197-206 (1999). In such a system, a plurality of column pixel driver circuits receive a common ramp signal which is repeatedly generated, during a plurality of cycles, by the output buffer of a digital-to-analog converter (DAC). Each column driver is coupled to all the pixels in a column of the electro-optic display device. During each ramp cycle, the column driver applies a prescribed voltage, corresponding to a desired pixel brightness level, to a pixel in a particular row in the respective column. The pixels in a column are selected by a row control circuit which selects successive pixel rows during successive ramp cycles.
Each of the column drivers, which receive the common ramp voltage from the DAC ramp generator, includes a track and hold circuit for storing the voltage signal when it reaches the value corresponding to the desired brightness level of a respective pixel. Since the voltage is sampled and stored on a capacitor in each driver, all the column drivers together represent a relatively high capacitive load to the ramp generator. When column sampling switches in the column drivers open or close, the transient load, in conjunction with the intrinsic output impedance and finite bandwidth of the output buffer amplifier of the ramp generator, can give rise to a voltage transient. A worst case condition occurs when a large number of column driver switches open simultaneously to sample a particular grey level (e.g., a background level) for the pixels in a particular row. In this case, the pixels in the same row, which are sampled immediately after this simultaneous sampling event (that is, at the next higher or lower grey level) will normally see the largest transient. In cases where the transient amplitude of the ramp signal is greater than the least significant bit (LSB) value, a visible artifact will be produced in the displayed image.
The voltage transients referred to above tend to increase as the resolution of the display is increased, since the switched capacitive load on the DAC ramp generator increases with an increasing number of column drivers. Also, as the display resolution (the numbers of columns and rows) increases, the switching frequency must increase to maintain the same frame rate. A higher switching frequency necessitates a lower DAC cycle time (the cycle period) resulting in even sharper transients. Higher frame rates, necessary to reduce flicker and color artifacts, call for a still further increase in switching frequency.
It is a principal object of the present invention to provide a color display system, having an analog electro-optic light modulator with a matrix of pixels, which employs a global ramp signal generator and individual track and hold column driver circuits for each column of the light modulator, and which reduces or eliminates voltage transients caused by switching capacitive loads.
This object, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by providing a compensation mechanism which enables a continuous current to be drawn from the ramp generator, and in particular the buffer amplifier of the ramp generator, independent of the current drawn by one or more of the column drivers.
More specifically, the present invention relates to a particular device for applying various levels of voltage to individual columns of pixels in an electro-optic display having a matrix of pixels, arranged vertically in columns and horizontally in rows. This device includes:
(a) a signal source having an output, the signal source producing at this output a monotonically varying voltage signal (e.g., a ramp signal) and repeating this voltage signal during a plurality of successive periods or cycles;
(b) a plurality of column drivers, each coupled to the signal source output and to the pixels in a column of the display, each column driver including a track and hold circuit for storing the voltage signal when it reaches a value corresponding to a desired brightness level of a particular pixel in the respective column during a given cycle, each column driver drawing current from the signal source during a portion of the cycle when the voltage signal is present; and
(c) at least one current source coupled to the signal source output for supplying current to the signal source output, during a portion of the cycle when said voltage signal is present, which is approximately equal in magnitude to the current drawn by one or more the column drivers.
This device according to the invention insures that the current drawn from the signal source (ramp generator) remains approximately constant, during the portion of each cycle when the ramp voltage signal is present, independent of the current drawn by one or more of the column drivers.
The present invention, is based on the recognition that the voltage transients appearing at the output of the ramp buffer amplifier are not directly caused by a change in the capacitive load, but by changes in the ramp buffer amplifier""s output current. This problem is solved by the compensation device described above, which enables a continuous current to be drawn from the ramp buffer amplifier. Preventing a change in the amplifier output current thus prevents a transient from occurring and allows a smooth, monotonic ramp to be generated, in spite of the sampling thereof by the column drivers.
The compensating device for maintaining constant the current drawn from the signal source (ramp generator, or more particularly the ramp buffer amplifier) may be implemented in a number of different ways.
According to a first preferred embodiment of the present invention, a plurality of current sources are provided, each associated with a separate one of the column drivers. In this case, each current source is operative to compensate for current drawn by its associated column driver.
Compensation may be achieved in one embodiment wherein the current drawn by each current source is made approximately equal to but opposite in sign to the current drawn by the associated column driver, and is drawn during the same portion of each cycle, as when the current is drawn by the associated column driver. In another embodiment, the current drawn by each current source is made approximately equal to and equal in sign to the current drawn by the associated column driver, and is drawn during a portion of each cycle, when current is not drawn by the associated column driver.
In still another preferred embodiment of the present invention, a single adjustable current source is provided to compensate for a varying current drawn by the plurality of column drivers. In this case, the current source is controlled, to adjust the amount of current generated by the source, by means of a control circuit which selects the amount of current produced by the current source in dependence upon the number of column drivers which apply a capacitive load to the ramp generator.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.